Limestone west of I-35, Blackland clay east
I-35 runs close to the Balcones Escarpment, where the Hill Country drops off into the Blackland Prairie. West of the highway, out toward Brushy Creek and Sam Bass Road, the soil is thin over limestone. East of it, past Red Bud Lane and out along the county roads, it is deep black clay.
Those prairie soils are the Houston Black kind. USDA records call them very slow to drain, with cracks up to four inches wide when the ground dries out. Water pours in through the cracks, then the clay swells and shuts. Texas A&M AgriLife Extension puts it plainly: only nonstandard drain fields can be used in clay.
West of the highway the trouble is the opposite. Shallow soil over rock leaves less room for a drainfield to work. Thin rock and heavy clay fail differently, and both turn up in drain field repair work here. Of the 490 new systems Williamson County authorized in 2025, only 17 were standard trench or bed drainfields.
Who permits a septic system in Round Rock?
The Williamson County Engineer's Office in Georgetown handles it. TCEQ named that office the authorized agent for septic systems across Williamson County. Applications go in online through MyGovernmentOnline. Round Rock also reaches into Travis County, so your address decides the office, not the city name.
Texas requires a permit to build, alter, repair or extend a septic system. There is a narrow emergency repair exception, and the county has to be told in writing within 72 hours. It covers tank lids, inlet and outlet devices, risers and riser caps, and solid lines. Work past that list starts with the county, and we handle that paperwork with you.
New lots on septic in Round Rock's ETJ run into the county's subdivision checklist. Lots using an on-site sewage facility have to be at least one acre. Lots without organized water have to be at least two acres. The checklist also makes the surveyor state whether the tract sits in the Edwards Aquifer Recharge Zone. Lots over that zone need a system designed by a professional engineer or registered sanitarian.
How often should a Round Rock tank be pumped?
TCEQ recommends pumping a conventional tank every three to five years. EPA lands in the same place, and adds that a household system should be inspected at least every three years. The real number for your house depends on tank size and how many people live in it.
Texas A&M AgriLife Extension put numbers on it. A 1,000 gallon tank is the standard size for a three bedroom Texas house. With four people in the house, that tank is due about every 2.6 years. With two people it stretches to about 5.9 years, and a garbage disposal pulls it back in.
The run over from Leander is RM 1431 east through Cedar Park, and it ends at I-35 in Round Rock. Before we come out we ask where the lids are. Texas rules keep tank lids within about a foot of grade unless risers were installed. On most properties that means shallow digging. When the lids are buried deeper, a septic tank riser brings them up to grade. That takes the digging out of every future visit.
Buying or selling a house on septic around Round Rock
Most Round Rock listings are on city sewer. A septic system in the contract usually means an acreage lot or a place out in the county. Texas does not require a septic inspection at a sale. The seller's disclosure has a box for a septic system, and lenders sometimes ask for an evaluation.
The option period is the window to find out what you bought. A real estate septic inspection is done with the tank pumped, because a full tank hides the parts that matter. We request the county's permit file, which in Williamson County comes through an open records request. The written report lands within 24 hours of the site visit.
Older systems are where people get caught. Texas exempts systems installed before September 1, 1989 from the current permitting rules. That holds as long as the system was never altered and is not failing. On a house that old the permit file can be thin. What is actually in the ground is the answer.
Communities and areas we serve around Round Rock: Brushy Creek, Palm Valley, Sam Bass Road, Brushy Creek Road, Great Oaks Drive, Hairy Man Road, Chandler Road, Red Bud Lane, Limmer Loop, County Road 110, County Road 112, McNeil Road.
Round Rock ZIP codes served: 78664, 78665, 78681.